Bussing is commonly used to interconnect various elements of a computer system. Typically each bus services two, three, or more devices. Single-ended bussing typically has a single set of data lines, one for each bit, with associated reference grounds, control, and power lines, all connecting to each device on the bus. Differential bussing typically has a differential pair of lines for each data bit, with associated reference grounds, differential pairs of control lines, and power lines, all connecting to each device on the bus.
Repetitive signals, including bus waveforms can be represented as a superposition of sine waves. A typical bus waveform has frequency components at a fundamental frequency equal to one-half the maximum transition rate of the bus, plus components at various harmonics of this frequency. Typical bus waveforms have significant energy in higher harmonics.
Common problems in bussing include reflections on, and crosstalk between, lines of a bus. Reflections and crosstalk can degrade the integrity of signals on the bus, leading to errors.
Typically, a bus line can be regarded as a transmission line carrying a signal. It is common to minimize reflections through termination resistors at each end of the bus. The magnitude of reflections at bus ends is a function of impedance matching between termination impedances to a characteristic impedance of the bus, and may be frequency dependent. Reflections may also arise from stub lines where devices tap into intermediate points along the bus, since stub lines are rarely terminated with termination resistors. The superposition of reflections on a bus is a function of frequency as well as the lengths of the stub and bus lines.
It is known that the lengths of the stub and bus lines cause resonances in the frequency response of the bus. This is because reflected signals have delay dependent upon these lengths. Reflected signals may reflect again from any point along the bus at which an impedance mismatch occurs. Each reflected, or re-reflected, signal has a particular phase relationship with respect to the original signal. Multiple delayed waveforms add or cancel at particular points along bus and stub according to the phase relationships of the original signal and each reflection or re-reflection present on the bus. These phase relationships are known to be a function of bus and stub lengths.
These resonances in bus performance may be very significant at particular harmonics of signals present on the bus, and much less significant at the next higher or lower harmonics.
Crosstalk is a consequence of mutual inductance and capacitance between near or adjacent bus lines. As such, crosstalk is strongly frequency dependent. In general, crosstalk tends to increase with increasing frequency since capacitive and inductive coupling increase with frequency.
Since reflections and crosstalk are frequency dependent, and can significantly degrade noise margin, it is desirable to minimize high frequency components of bus waveforms. Controlling particular high frequency components minimizes reflections along, and crosstalk between bus lines, thereby avoiding errors in a system.
Integrated circuit design and fabrication not only is very expensive, but design times are often much longer than system board design times. Integrated circuits may be used on multiple, somewhat different, system boards. Bus and stub lengths vary with system board design.